In recent years, portable information terminals such as cell phones have become known. These are provided with a geomagnetic sensor for detecting geomagnetism, and conduct directional measurement based on the geomagnetism detected by this geomagnetic sensor. The measured directions are used, for example, in the display of maps. To cite one example, cell phones are appearing that possess the function of displaying maps according to the cell phone orientation (direction) based on current positional information obtained by GPS (Global Positioning System), which conducts position detection.
Incidentally, the properties of geomagnetic sensors vary according to the chip, and these properties should be corrected by some type of means. For example, the circle drawn from the output when a cell phone incorporating a geomagnetic sensor, which possesses magnetism detection directions along two axes (X-axis and Y-axis directions) in a horizontal plane as the sensitivity directions, is slowly rotated one or more revolutions at uniform speed in a fixed magnetic field without altering the horizontal state is referred to as the azimuth circle.
This type of azimuth circle is ideally centered on the origin of the intersection of the X axis and Y axis, and possesses a specified radius. However, as mentioned above, the properties of geomagnetic sensors vary according to the chip, and a magnetic field exists inside the cell phone. Due to the existence of this variation in properties and the aforementioned magnetic field, the center of the pertinent azimuth circle shifts from the origin. This shift is referred to as the offset, and this shift value is referred to as the offset value. When this type of offset exists, the direction calculated based on the measurement value of the geomagnetic sensor on the assumption that there is no offset differs from the actual direction. Consequently, the geomagnetic sensor corrects the pertinent offset from the measurement value.
As correction of the offset from the measurement value of the geomagnetic sensor is here conducted by subtracting the offset value—obtained by digital computation from a plurality of measurement values obtained by rotating the pertinent cell phone—from the measurement value, it is necessary to store the offset value used for the pertinent correction in the geomagnetic sensor. Consequently, a conventional geomagnetic sensor is configured on one chip by combining a geomagnetic sensor element for detecting geomagnetism (magnetic field) with an arithmetic unit for calculating the offset value from the measurement value of the pertinent geomagnetic sensor element, an A/D converter for conducting A/D (analog/digital) conversion of the pertinent offset value, and an EEPROM (Electronically Erasable and Programmable Read Only Memory) for storing the offset value that has undergone A/D conversion.
However, in conventional geomagnetic sensors, the EEPROM that stores the aforementioned offset value applies a thin oxide film like a tunnel insulating film to the memory unit, and laminates the polysilicon layer and metal layer that serve to form the bit lines and word lines on the chip into several layers. Consequently, a special process is required to manufacture the pertinent chip, resulting in the problem of a high unit price for the chip. Moreover, EEPROM requires a high voltage generation circuit, a writing circuit and the like for purposes of writing, which results in the further problem that chip size is enlarged, and the geomagnetic sensor system for driving EEPROM grows larger in scale.
Conventionally, in the temperature sensor formed on the semiconductor chip, one uses, for example, the temperature sensor circuit 212 shown in FIG. 12. The temperature sensor circuit 212 is composed of the op amp OA, the diodes D1-D2, the resistors R1-R3, and the A/D (analog/digital) converter ADC. The op amp OA, diodes D1-D2 and resistors R1-R3 configure a general band gap reference circuit.
The noninvertible input terminal of the op amp is connected to the anode of the diode D1, and the cathode of the diode D1 is grounded. The invertible input terminal of the op amp OA is connected to one end of the resistor R3, the other end of the resistor R3 is connected to the anode of the diode D2, and the cathode of the diode D2 is grounded. The output terminal of the op amp OA is connected to the noninvertible input terminal via resistor R1, and connected to the invertible input terminal via resistor R2.
The output terminal OpVref for outputting the output voltage Vref of the pertinent band gap reference circuit is connected to the output terminal of the op amp OA, while the output terminal OpVbe for outputting the pertinent voltage Vbe to diode D1 is connected to the noninvertible input terminal of the op amp OA.
Output terminal OpVref of the pertinent band gap reference circuit is connected to input terminal IpVh of the A/D converter ADC, and output terminal Opbe of the pertinent band gap reference circuit is connected to input terminal IpV1 of the A/D converter ADC. The A/D converter ADC conducts A/D (analog/digital) conversion of the differential voltage V of the voltage inputted to input terminal IpVh and the voltage inputted to input terminal IpV1, and is provided with output terminal OpDout for outputting the output value Dout, which is the conversion value of the output voltage. The aforementioned A/D converter ADC sets the range of the voltage Vin at 0-1.25V, conducts A/D conversion of the pertinent voltage Vin at 1251 step (0-1250), and outputs the output value Dout.
Next, the operations of the temperature sensor circuit 212 are explained. The band gap reference circuit comprising the op amp OA, the diodes D1-D2 and the resistors R1-R3 outputs the output voltage Vref (=1.25 V), which has little source voltage and temperature dependency, from output terminal OpVref to input terminal IpVh of the A/C converter ADC, and outputs the output voltage Vbe that has a temperature coefficient of approximately −2 mV/° C. from output terminal OpVbe to input terminal IpV1 of the A/D converter ADC. The A/D converter ADC then conducts A/D conversion of the voltage Vin, which is the differential voltage of the output voltage Vref and the output voltage Vbe, at 1251 step, and outputs the output value Dout.
At this time, as the output voltage Vref has little source voltage and temperature dependency, it can be treated as a constant, and as the output voltage Vbe has a temperature coefficient of approximately −2 mV/° C., the voltage Vin varies according to temperature corresponding to the output voltage Vbe. Accordingly, the output value Dout varies according to temperature. In this context, if one assumes that the output voltage Vbe is 0.6V when the ambient temperature is 25° C., the output value Dout will be as shown in the following formula (21).Dout=−2(T−25)+600  (21)According to formula (21), when ambient temperature is 30° C., the output voltage Vbe is 0.59, and the output value Dout is 590.
As literature relating to this invention, Japanese Patent Application 2004-85384, for example, records a temperature sensor circuit wherein the partial pressure ratio of the resistor group is adjusted by a fuse circuit combining the fuse and the pertinent resistor group, where the influence of manufacturing process variations can be eliminated, and high-precision temperature compensation can be conducted by means of the output voltage adjusted by the pertinent fuse circuit.
However, in the aforementioned temperature sensor circuit 212, with regard to the output voltage Vref of the internal band gap reference circuit and the pertinent voltage Vbe in diode D1, the voltage values and temperature properties have individual variations, resulting in the problem that it is difficult to improve the accuracy of the output value Dout, which is a measurement value.
In general, LSIs (large-scale integrated circuits) that mount a magnetic sensor for two transverse axial directions on the chip to conduct geomagnetism detection possess a means for correcting the sensitivity of the geomagnetic sensor.
As a technology that conducts correction of the detection output of a magnetic sensor by arithmetic processing, there is, for example, the one recorded in Japanese Patent Application 2000-180170. According to the technology recorded in this same literature, correction of the detection output of the X axis detection portion is conducted as follows. That is, the detection range of the magnetic sensor is divided into 4 blocks at 90° each, the maximum output voltage value of the X axis detection portion is A1, and the output voltage value of the X axis detection portion at the point where there has been a 90° rotation from the position where the output value of the Y axis detection portion is zero is A2.
This is classified according to cases where the output voltage value A2 is on the + side, cases where it is on the − side, and cases where it is infinitesimal. In the case of the + side, formula (301) is applied as the correction formula; in the case of the − side, formula (302) is applied as the correction formula; and in the case where infinitesimal, no correction is made.[ABS(A3)+ABS(A2)]·Z  (301)[ABS(A3)+ABS(A2)]/Z  (302)Provided that A3 is the actually measured output of the X axis detection portion, and Z is the correction parameter shown in formula (303).Z=A1/[A1−ABS(A2)]  (303)Correction is conducted by the same technique for the Y axis as well, and the degree of orthogonality of the X-axis detection portion and the Y-axis detection portion is corrected.
In the case where correction of the detection output of the magnetic sensor is conducted by arithmetic processing in this manner, one may, for example, adopt the mode where correction data is measured in shipment inspection, and written into a nonvolatile memory mounted in the LSI.
Incidentally, with regard to this type of LSI, one has recently become available that mounts a fuse memory for receiving requests for low voltage generation, and for enabling appropriate reading as nonvolatile memory even at low voltage.
However, in contrast to the aforementioned advantages, the fuse memory requires large capacity in the transistor for the fuse cutting that is used during writing, and it is necessary to pay attention to the scale of the circuit. Consequently, with the mode where correction data values obtained at the time of shipment inspection are written into the fuse memory without alteration, numerous fuse memories are required, which is inconvenient for purposes of circuit design.